I dont think the problem is with 'have you bought' or 'did you buy' the problem is with the use of 'the' it should be in 'a' new dress regardless of how you phrase the first part of the question
Had you bought a new dress is not correct. It is not a question. Had you bought a new dress you could wear it. This would be correct.
Seeing Trofaste's answer, I'm keeping it, but it will not be a suggested option anymore, just an accepted one.
That's certainly a correct sentence, but "Had you bought a new dress?" is correct as a question too. Remember it's past tense, not present tense. You can say "You had bought a new dress before going to the party", then just turn it around to make it a question, "Had you bought a new dress before going to the party?" Make sense?
Actually, "Had you bought a new dress?" is not a correct sentence. The past perfect is used when two actions take place one before the other. 'Had you bought a new dress, you could wear it to the party.' Fine. "Had you bought a new dress before going to the party?" No, sorry grammatically incorrect, and does not make sense. Why would you say "You had bought a new dress before going to the party"..? 'You bought a new dress... Did you buy a new dress... 'had' is not correct English. I like this application, and I know that many people use it to learn English and other languages at the same time. That is why getting the English correct is important. It would also be nice if there was a British English option too!
I speak British English. :)
Do you have any sources for your statement that "Had you bought a new dress before going to the party?" is grammatically incorrect? I'd be quite interested in seeing them.
You said "The past perfect is used when two actions take place one before the other." My sentence is one action taking place before the other... 1) had you bought a new dress, before 2) going to the party? You also said "[it] does not make sense." There are many different varieties of English and even British English isn't the same throughout the country, and it makes perfect sense to me and plenty of other natives. You don't use it, I accept that, but that doesn't make it incorrect everywhere else as well.
I'd like to link to the Cambridge Dictionary's page on the past perfect. Note that I'm not talking about the past perfect in conditional clauses; this isn't a conditional clause. I'm talking about the first category in Past perfect simple: uses, "Time up to then".
Do you mean :
you had multiple choice, and past perfect was one of them. While we used to have equivalent of past perfect, we do not use it any more, we use past tense for things that happened before other past things.
why is the lesson called past perfect? It's misleading, it should be past tense of perfective verbs or sth like that.
Strange... There were words to rearrange into a sentence and as far as I remember they offered 'had' and 'bought'
Did you buy a new dress?
Have you bought a new dress?
Had you bought a new dress?
are valid translations depending on context.
I think I would go with Have you bought a new dress? as my first assumption.